<!DOCTYPE html>
<html>
  <head>
    <style>

input {
  width: 15em;
}

input[type=number]::placeholder {
  color: blue;
  opacity: 1.0;
}

input[type=number]::-moz-placeholder {
  font-style: italic;
  opacity: 1.0;
}
    </style>
  </head>
  <body>
    <input type="number" placeholder="123">
  </body>
</html>
